1. Field of the Invention
The present invention generally relates to an apparatus and method for generating ultrashort optical pulses at a plurality of optical wavelengths, and, more particularly, to an apparatus and method using optical fibers and optical waveguides to produce and control such optical pulses. Ultrashort is here generally referred to as being within the time scale of approximately 10xe2x88x9215 seconds (femtoseconds) to 10xe2x88x9212 seconds (picoseconds).
2. Description of the Related Art
A variety of laser systems for producing ultrashort optical pulses is known in the prior art. From a practical point of view, these systems can be generally grouped into two main categories: solid-state laser systems, which are based on the use of volume laser gain media, and fiber laser systems, which are based on waveguiding fiber-optic components. Due to their intrinsic structure, fiber lasers have a number of basic properties which make them significantly more suitable for widespread practical use. As is well known in the prior art, fiber lasers are compact, can be diode pumped, and are robust and reliable. For a number of reasons, at present, the most mature technology suitable for ultrashort-pulse fiber laser systems is based on Er-doped fiber providing output pulses having a wavelength of approximately 1.55 xcexcm. First, Er-doped fibers are among the best developed of the rare-earth-doped fibers. Diode lasers for pumping such fibers are also well advanced.
Significantly, the generation of ultrashort pulses requires design-control of the dispersion in the laser cavity. This can be accomplished in a compact, all-fiber cavity only at wavelengths above 1.3 xcexcm, where the dispersion of the optical fiber can be tailored to be either of positive or negative sign. However, a variety of practical applications for ultrashort pulses require other wavelengths of operation, for example, either at shorter or longer wavelengths. At those wavelengths, femtosecond-pulse fiber oscillators at present can be designed only by using bulky external components, such as sets of prism pairs, to control the in-cavity dispersion.
The general and well known method to extend the wavelength range of any particular laser system is to utilize nonlinear optical interactions, such as optical harmonic generation, sum or difference frequency generation and optical parametric gain.
Harmonic generation is suitable only for converting an optical signal to a higher optical frequency (shorter wavelength) and it cannot provide tunable or multiple-wavelength output. Sum-frequency and difference-frequency generation allows conversion of a signal to both higher and lower optical frequencies and allows wavelength tunability, but requires at least two well synchronized optical sources at two different optical frequencies. Therefore, each of these interactions alone cannot provide multiple-wavelength or wavelength-tunable output from one, single-wavelength signal source.
Optical parametric interaction is suitable for providing tunable or multiple-wavelength conversion using one, single-wavelength optical signal source. Furthermore, while optical parametric conversion allows conversion of an optical signal only to a lower optical frequency (longer wavelength), by combining parametric interaction with at least one of the above described interactions, any optical frequency above or below the signal-source frequency can be obtained.
The general drawback of parametric optical frequency conversion is that, in order to achieve high parametric gain sufficient to amplify spontaneous quantum-fluctuation noise from microscopic to macroscopic levels and, consequently, to achieve efficient signal-energy conversion, high peak-powers and high pulse-energies are required. It is well known from the prior art that the required energies are well above the energies that can be generated directly from a typical mode-locked, ultrashort-pulse laser oscillator. The best demonstrated result known to date is an optical parametric generation (OPG) threshold at xcx9c50 nJ, and efficient OPG conversion of xcx9c40% at approximately 100 nJ achieved in bulk periodically-poled lithium-niobate crystals, as reported by Galvanauskas et al. in xe2x80x9cFiber-laser-based femtosecond parametric generator in bulk periodically poled LiNbO3xe2x80x9d; Optics Letters, Vol. 22, No. 2; January, 1997. In comparison, typical femtosecond mode-locked pulse energies from a fiber laser are in the range of 10 pJ to 10 nJ (as described by Fermann et al. in xe2x80x9cEnvironmentally stable Kerr-type mode-locked erbium fiber laser producing 360-fs pulsesxe2x80x9d; Optics Letters; Vol. 19, No. 1; January, 1997, and by Fermann et al. in xe2x80x9cGeneration of 10 nJ picosecond pulses from a modelocked fibre laserxe2x80x9d; Electronics Letters, Vol. 31, No. 3; February, 1995) and those from a solid-state laser are in the range of up to xcx9c30 nJ (as described by Pelouch et al. in xe2x80x9cTi:sapphire-pumped, high-repetition-rate femtosecond optical parametric oscillatorxe2x80x9d; Optics Letters, Vol. 17, No. 15; August, 1992).
It is known from the prior art that efficient optical parametric wavelength conversion can be achieved with unamplified or amplified mode-locked laser pulses by arranging a nonlinear crystal in a separate optical cavity in a manner that ensures that pump pulses and signal pulses pass the parametric gain medium synchronously, as seen, for example in the above-referenced article by Pelouch et al. Since, in this case, parametric interaction occurs repetitively, the low, single-pass parametric gain and, consequently, low pulse energies of mode-locked oscillators are sufficient to achieve efficient conversion. The significant practical drawback of this approach is that such a scheme requires two precisely length-matched optical cavities; one for a mode-locked oscillator and another a for synchronously-pumped optical parametric oscillator (OPO). Consequently, such OPO systems are complex, large, and intrinsically very sensitive to the environmental conditions (non-robust). Furthermore, wavelength tuning of such a system requires mechanical movement of the tuning elements such as rotation or translation of a nonlinear crystal, rotation of cavity mirrors, etc., which is incompatible with fast wavelength tuning or switching. Therefore, OPOs can not serve as practical ultrashort-pulse sources for producing multiple-wavelength pulses directly with mode-locked oscillator output.
It is a general object of the present invention to provide a method and apparatus for generating ultrashort optical pulses at a variable or adjustable optical wavelength from a single source which provides ultrashort optical pulses at a fixed optical wavelength.
It is a further object of the present invention to provide a method and apparatus for generating ultrashort optical pulses at a plurality of optical wavelengths using a single source which provides ultrashort pulses at a fixed optical wavelength.
Another object of the present invention is to provide fast control of the output of a laser system in order to select between a plurality of wavelength conversion channels.
Still another object of the present invention is to provide a plurality of wavelengths at the single output of a laser system by combining outputs from separate wavelength-conversion channels into a single output beam.
Yet another object of the present invention is to enable efficient multiple-wavelength or adjustable-wavelength operation at relatively low pulse energies and powers which are compatible with existing ultrashort-pulse laser oscillators. An additional object of the present invention is to implement such a system using components which are robust, compact and well-suited for large-volume fabrication in order to provide a compact, robust, easily manufacturable and cost-effective apparatus.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, these objects are achieved in a system having a first part comprising a laser system for producing ultrashort pulses at a fixed wavelength, and a second part comprising at least one and preferably a plurality of wavelength-conversion channels. A wavelength-controlling element (or elements) is disposed between the laser generator and the wavelength-conversion channels, which element(s) directs the pulses from the laser generator into at least one of the wavelength conversion channels. Another component or plurality of components is disposed downstream of the wavelength-conversion channels and serves to combine outputs from separate wavelength-conversion channels into a single output channel.
According to the present invention, novel optical waveguide devices are used for the wavelength-conversion channels, wavelength-control and beam-control elements. Preferably, a fiber laser system is used for generating single-wavelength, ultrashort pulses. The multiple-wavelength laser system of the present invention advantageously replaces a plurality of different, single-wavelength laser systems.
One application for the present invention is in systems that require ultrashort optical pulses at wavelengths that are different from the wavelength of the pulse-generating laser. For example, the system of the present invention can shift the ultrashort pulse wavelength to approximately 1.3 xcexcm for optical coherence tomography (OCT), where tissues are most transparent.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.
All of the above-referenced articles are incorporated herein by reference in their entirety.